1. Field of the Invention
The present invention relates to drilling fluids and methods useful in drilling boreholes in water-sensitive formations. More particularly, this invention relates to water based drilling fluids and methods useful for stabilizing water sensitive shale formations in oil and gas drilling operations.
2. Description of Relevant Art
For many years the oil industry has searched for effective or highly inhibitive water-based drilling fluids that could be successfully substituted for oil-based drilling fluids for use in drilling water-sensitive formations such as swelling or sloughing shales, salt, gypsum, anhydrite or other evaporate formations, hydrogen sulfide-containing formations, and hot (greater than about 300 degrees Fahrenheit (“° F.”) holes. Although oil-based drilling fluids, most commonly invert emulsion fluids, have generally performed well as drilling fluids for such water-sensitive formations, water-based fluids are expected to be generally more economical and overall more readily compliant with environmental regulations. Moreover, in certain formations, water-based fluids are believed more suitable than oil-based fluids, even though such formations contain water-sensitive zones.
Shale hydration, however, commonly seen when ordinary water-based fluids are used in water-sensitive formations, is a significant cause of wellbore instability. Thus, any drilling fluid used in shales must exhibit minimal reactivity with the shales. That is, the water in the fluid should not cause swelling of the clays in the shales.
Various water-based systems have been tried with varying amounts of success. Potassium/PHPA (polymer) fluids are one example, but these generally cannot reach the inhibition levels of oil-based fluids in water-sensitive clays. Cationic polymer fluids are another example, but these tend to be incompatible with anionic drilling fluid additives and may sometimes have toxicity problems.
High molecular weight polyacrylamide polymers are commercially available and well known for their ability to impart borehole stability by inhibiting shale hydration. An example is CLAY GRABBER® product, a high molecular weight polyacrylamide material available from Halliburton Energy Services, Inc. in Houston, Tex. While helpful, high molecular weight polyacrylamide polymers still do not provide the wellbore and shale stability advantages of an oil based drilling fluid and cannot impart to a water based drilling fluid the lubricity and resistance to contaminants that an oil based drilling fluid typically has. Additionally, high molecular weight polyacrylamide polymers cannot impart to a water based drilling fluid the ability to achieve the fast penetration drilling rate in the formation that can be obtained with an oil based drilling fluid. There is still a desire for a water-based drilling fluid with improved performance in water-sensitive formations or zones that can approximate the performance advantages of an oil based drilling fluid.